London's green belt could be sacrificed to Los Angeles-style urban sprawl in the name of economic growth under sweeping reforms to the planning system unveiled by the government this week, the National Trust has warned.

The 3.6 million-member organisation voiced "grave concerns" on Tuesday over government proposals to slash 1,000 pages of planning policy to just 52 pages in a move that has won the ringing endorsement of property developers.

Opponents claim the new draft policy effectively removes the national target for recycling brownfield land and allows local communities to support building on the green belt. It is set to be the biggest change to the planning system in more than 60 years and scraps detailed planning guidance notes and circulars. Instead, the government insists there should be a presumption in favour of "sustainable development" to house a rising population.

The national planning policy framework (NPPF) is intended to speed up and simplify often complex laws at the same time as encouraging economic growth. In a foreword to the new policy, Greg Clark, the minister for decentralisation, said: "We must accommodate the new ways by which we will earn our living in a competitive world ... Development that is sustainable should go ahead, without delay."

Fiona Reynolds, director general of the National Trust, warned the policy could lead to unchecked sprawl in the countryside on a scale not seen since the 1930s.

"The government's proposals allow financial considerations to dominate and with this comes huge risk to our countryside, historic environment and the precious local places that people value," she said.

"This finally sounds the death knell to the principle established in the 1940s that the planning system should be used to protect what is most special in the landscape."

The new policy was drawn up with the help of a "practitioners advisory group", members of which included a Conservative councillor, a director of the housebuilding firm Taylor Wimpey, a planning consultant who represents major developers at government level and an official from the Royal Society for the Protection of Birds.

"By replacing over a thousand pages of national policy with around 50, written simply and clearly, we are allowing people and communities back into planning," said Clark.

But the changes to planning appear likely to produce a popular backlash, according to conservationists. The National Trust is asking its members to sign a petition urging ministers to reconsider.

Shaun Spiers, chief executive of the Campaign to Protect Rural England, said the government appeared to have declared "open season" on countryside not designated as green belt or an area of outstanding natural beauty.

"The new framework will make the countryside and local character much less safe from damaging and unnecessary development," he said. "If it is not amended, there will be battles against development across the country that will make the public revolt against the sale of the forests look like a tea party."

The Campaign for Better Transport and Friends of the Earth have also attacked the plans.

Campaigners said if the proposals were not amended they could backfire on the government in the same way that plans to sell off the forestry estate in England earlier this year resulted in an embarrassing U-turn.

Ominously for the government, many of the same groups who opposed the sale of the forests are at the forefront of the criticism of the draft NPPF, published on Monday by the Department for Communities and Local Government. Most of the groups objecting have strong representation in Tory-held constituencies.

"It's refreshing to have a concise national planning framework which supports and encourages growth, and at same time protects our heritage," said Francis Salway, chief executive of Land Securities.

"The focus on economic growth is very welcome, while also acknowledging the important role that planning has in protecting the environment," said Chris Grigg, chief executive of British Land. "We particularly welcome the presumption in favour of sustainable development."